Production of optical recording media having plural luminescent recording layers by embossing the recording layer

ABSTRACT

Processes for producing the multi-layered optical recording medium shown in FIG. 2 using the embossing master (20) shown in FIG. 3, where an etched metal thin layer (22) has been electroplated with a thick metal layer and the luminescent recording layer (25) cast thereover forming pits (24) between adjacent metallized areas. The luminescent layer may then be cured using UV light (26).

This is a National Phase Application of PCT/US99/05857 filed Mar. 18,1999, which in turn claims priority of U.S. Provisional ApplicationSerial No. 60/078,514, filed Mar. 18, 1998.

FIELD OF INVENTION

The present invention relates to the production technology of theCD-ROM-, WORM-, WER-class multi-layered optical discs. Moreparticularly, it refers to the methods of replication of themulti-layered optical discs by using the relief-carrying master-discsproduced by the technology similar to the contact lithography, as wellas to the relief-carrying master-discs with various wetting propertieson their surface and in the micro-cavities (pits and grooves), as wellas to the liquid light-sensitive compositions of the informativeluminescencing media for implementation of the said methods.

The inventive method has advantages, which enable the production ofpolychrome discs, and thus increase the information. capacity of thediscs a few times (approximately, by the number of times equal to thenumber of chromatic components utilized in a polychrome disc).

References US and other foreign patent documents 3,946,367  3/19764,219,704  8/1980 4,450,553  5/1984 4,905,215  2/1990 4,908,813  3/19905,063,556 11/1991 5,202,875  4/1993 5,251,198 10/1993 5,255,262 10/19935,373,499  3/1995 5,381,401  1/1995 5,408,453  4/1995 5,468,324 11/19955,526,338  6/1996 5,540,966  7/1996 5,555,537  /1996 5,669,995  9/19974,451,914  /1984 4,829,505  /1989 4,908,813  /1990 5,134,604  /19925,175,720  /1992 5,218,599  /1992 5,220,556  /1993 5,285,974  /19935,513,170  /1996 5,511,057  /1996 5,526,336  /1996 5,526,338  /19961078060  5/1980 Canada 352/32 0461956 12/1991 European Patent OfficeG11B 7/00 60-202545 10/1985 Japan G11B 7/00 62-271236 11/1987 Japan G11B7/14 63-276732 11/1987 Japan G11B 13/00 63-3116548  5/1987 Japan G11B7/09

Other Publications

1. N. K. Arter et al., “Optical Disc Family”, IBM Technical DisclosureBulletin, 30, N2, p.667, 1987.

2. S. Brown, “The decade of the CD”, San Jose Mercury News, p.8, Jan. 9,1994.

3. Y.Okino et al., “Developments in fabrication of optical discs”,Optical Disc Technology, p.236, 1982.

4. S. Horigome, “Novel stamper process for optical disc”, OpticalStorage Technology, p.121, 1988.

BACKGROUND

Multi-layered fluorescent optical discs can find wide use owing to thepossibility of reaching high bulk density when recording information onthem, as well as of retrieving the information with high contract andsignal-to-noise ratio.

For this purpose it is desirable to locate a luminescence-emittinginformation medium only in the pits of CD-ROM or in the pre-recordinggrooves of CD-WORM or CD-WER. In the present invention, this problem isresolved by using the photolithography technology, or specific types oforiginal master-discs.

Use of contact photolithography to record elements with the resolutionas defined in standards for the polycarbonate-base CD is ratherdifficult, since it is necessary to provide spaces less than 0.2μbetween the photomask and disc surfaces. The disc surface has aroughness of the order of few microns; moreover, disc thickness can varygradually by tens of microns. As a result, it is impossible to press thedisc to photomask uniformly all over the surface.

By means of modern lithographic devices with adaptive-optics projectors,it is possible to create the required recording elements, but only noton very large disc surfaces; in addition, the projector optics must beadjusted separately when recording each region, and that requires a lotof time and creates problems with recording at the borders of suchregions. Besides, these devices are very expensive.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a partial cross-section view of the multi-layered opticalfluorescent disc as a first embodiment of the present invention (in thecase when the layer of information luminescing medium (hereinafterreferred to as ILM), covering the flat surface of the photomask, isremoved (washed away) after the exposure).

FIG. 2 is the same as FIG. 1, but when the ILM layer is not removed andcontains no photo-chemically inactive luminophor.

FIG. 3 is a schematic view of the master-disc used for replication ofsingle-layered optical discs.

FIG. 4 is a schematic diagram of the master-disc as the secondembodiment of the present invention.

SUMMARY

In one embodiment of the present invention, the master-disc is made notin the form of a one-piece solid metal disc with pits or pre-recordinggrooves, but as a metallic-coated photomask formed on a transparentbase, and by using the photolithographic methods for forming pits andpre-recording grooves.

I. Methods of Production of Thin Information Layers

The essence of the methods consists in applying an information layerdirectly on photomask with a subsequent removal from it after exposurethrough metal (1) or without exposure (2). This enables getting rid ofthe diffraction-related spreading of element images and theirnon-uniformity over the surface, which are inherent in contactphotolithography and are caused by a gap between photomask andphotoresist due to a roughness of their surface or non-uniformity whenpressing down. The application of the information layer can be carriedout by various methods, including spin-coating.

As the information layer, the following systems can be used:

1. A fluorescent dye in a light-sensitive composition (photo-polymer,oligomer, monomer). When exposed to UV-radiation, polymerization takeplace only in pits (grooves), then non-polymerized sites are removed bywashing them off by means of appropriate solvents.

2. The mixture of low-molecular polymer with photo-active dye. As thephoto-active dyes, the following substances may be used:

a) Lactams [2] and lactones of rhodamines and other organic dyes [3].When exposed to λ₁-wavelength radiation, they pass to the coloredstructure which absorbs the radiation at the λ₂-wavelength and emitsfluorescence rays on the λ₃-wavelength;

b) Industrially manufactured photo-affined fluorescent labeles attachedby the UV radiation to polymer molecules, for example, dye azides suchas cascade blue or lucifer yellow AB and so on [4], which can befastened to amine groups of gelatin or other polymers containing thesegroups, and so on with a subsequent washout of non-fastened dyemolecules.

Therefore, applying the information layer directly on the photomaskenables minimization of dimensions of record elements and increase ofdisc information capacity; in addition, a simple process of exposurereduces the cost of a finished disc.

1. Production of Information Layer by the Photomask Method

When transferring the image of the information elements from thephotomask into an information layer, it is very important to do thiswithout changing their dimensions and location. We propose the methodthat avoids the distortions related to diffraction of light on thephotomask perforations using a “deep” photomask. Its thickness isdetermined by a number of circumstances.

When being applied, the polymer will fill those sites on the photomasksurface which are free of metal and will be poured in from above as athin layer, thus filling in all the disc surface. When exposed frombelow, only the polymer sited inside the pit will pass into a coloredform. When using photomask with the correctly selected composition, thedepth of absorbed radiation in the material of the information layer 3(FIG. 2) at exposure wavelength λ₁ can be made equal approximately to athe metal thickness. As a result, it is possible to avoid the effects ofdiffraction and, therefore, to decrease the pit dimensions.

The increased depth (as compared with the standard one) is necessary forincreasing the information layer thickness to a possible maximum extent.It enables, under conditions of keeping the same amount of lightabsorption and, respectively, the number of molecules of dye(photo-chrome) in a pits, (a) to reduce heating by laser radiationduring reading since the concentration of the photo-active molecules ina large volume is lower, and (b) to reduce the probability ofaggregation of these molecules.

The dye concentration necessary to absorb 10% of incident radiation (ina linear range) for typical extinction coefficient (ε) is calculated bythe formula C=−(1/ε_(d))lgT, where T indicates the transmittance. Thedependency of the dye concentration, required to absorb 10% of thereading radiation in the pits with a depth of d=0.1μ and d=0.5μ, on theextinction coefficient at the wavelength of the reading radiation, isgiven in Table 1.

TABLE 1 ε (1/mole cm) d 10⁴ 2 × 10⁴ 5 × 10⁴ 10⁵ d = 0.1μ 0.458  0.228 0.0915 0.046  d = 0.5μ 0.0915 0.0456 0.0183 0.0092

With the molecular mass of the dye equal to 500, the dye must constitutefrom 22.9 to 2.29 percents (by volume) in the pits.

The given thickness of metal (the photomask depth) without loss ofresolution can be provided if the photomask is made initially with athin metal layer, the thickness of which is then enlarged byelectroplating.

2. Production of the Information Layer by Masking

It is proposed to use a master-disc which is made so that the pit bottomis wetted properly by the polymer of the information layer, and thesurface of the disc is not wetted; in addition, in order to improvewettability, a special sub-layer with a good wettability with respect tothe polymer may be applied on the bottom of the pit. In this case, asthe material for the information layer, the polymer with a fluorescentdye additive is used. By doing so, the solution of this polymer fills inonly holes (pits). With the purpose to make the filling of pits mucheasier and to guarantee that the spaces between pits is not covered bythe dye-containing polymer, the master-disc may be produced so that thebasic mass of the master-disc is made of the material which exhibits agood wettability with respect to the solution of the dye-containingpolymer, and the upper thin layer shows no wettability (see FIG. 4). Asshown in FIG. 4, this layer may have a thickness equal to the requireddepth of pits, i.e. when producing the master-disc, only the thin layerwill be etched, and the base of the master-disc will not be subjected toetching, but this layer may also be substantially thinner than therequired depth of pits, with etching up to the required depth already inthe material of the master-disc base. In any case, when producing pits,the through holes are etched in the thin layer. This thin layer is madeof the material exhibiting a good adhesion with the material of thebasic mass of the master-disc, but is not wettable by the polymersolution. After shaking off, blowing off, or blotting of the discsurface, on which the polymer solution with dye is applied, the drops ofsolution will remain only in the holes (pits) of the master-disc, andthe rest of the surface will contain no polymer solution with dye.

After being dried without removing completely the tracks of the solvent,the hardened islands of the dye-containing polymer film, which fill thepits, shall still keep a sufficient adhesion with the bottom of thepits.

B. Production of the Optical Disc by Sticking Thin Multi-layeredInformation Structures One to the Other Sequentially

On the information layer produced by the above-mentioned methods, aseparation layer of a thickness of 5-100 μm and made of a pure polymer,or a polymer with a substance capable of absorbing the UV-radiation, isapplied. This substance may be needed for protecting the dye againstbleaching on exposure to the UV-radiation, which may be used forhardening of the photo-polymer glue. The glue is applied on the surfaceof the separation layer by a thin layer, for example, by using acentrifuge. Then, by means of the applied glue, the resultantmulti-layered structure is glued to a mechanical base above themulti-layered information structures, applied previously; as themechanical base, a polycarbonate disc may be used.

Photo-method of producing the multi-layered optical fluorescent disc (bythe example of photo-active dyes):

1. The intermediate sublayer 2, which exhibits a transparency in the UVand visible spectrum sections, has the refractive index n₂ and thethickness d₂, and loses its adhesive properties with respect to thephotomask during drying, is applied on the photomask made of thematerial transparent in the UV andvisible spectrum regions, with theinformation recorded in the form of spiral tracks. This layer may becomeunnecessary if a variation in the adhesive properties of the informationlayer, applied directly on the photomask due to various types of drying,is sufficient and will enable the treatment of the layer without itspeeling from the photomask till the moment of the final drying andsticking to the carrying polycarbonate disc with the previousinformation and separation layers.

2. On this layer 2, the photoactive polymer layer 3 is applied in anoriginal form having the refractive index n₃ and the thickness d₃ andcapable of irreversible photo-conversion (for example, rhodamine lactamsor lactones) when exposed to the radiation on the wavelength λ₁, and thecolored form of which absorbs the radiation on the wavelength λ₂, andemits fluorescence on the wavelength λ₃, with λ₁<λ₂<λ₃.

3. Then, on layer 3, the polymer layer 4, having the refractive index n₄and the thickness d₄ and absorbing light at the wavelength λ<λ₄ andbeing transparent on λ₂ and λ₃, with λ₁<λ₄<λ₂<λ₃, is applied tosubsequently provide the protection of the exhibited layers from the UVradiation.

4. On layer 4 is applied layer 5, having the refractive index n₅ and thethickness d₅ and being transparent on λ₂ and λ₃.

5. The resultant multi-layered structure, if it is the first one, isglued on a mechanical base in the form of the standard optical, forexample, polycarbonate, disc 6 (FIG. 1), or, if this structure is then-th structure, it is glued on the half-finished product of the opticaldisc with n−1 multi-layered structures, with the 5th layer of the nthstructure being glued to the 2nd layer of the (n−1)th structure. If the2nd layer is missing, then the 5th layer is glued directly onto the 3rdlayer.

6. Polymerization or drying of the layers and exposure of the 3rd layerof each multi-layered structure are carried out at the most suitablemoment of the described technological process.

7. Removal of the nth resultant multi-layered structure from thephotomask is carried out at any moment after gluing to the (n−1)th one,for example, before gluing to it the (n+1)th multi-layered structure.

8. After the last photomask is removed, protection layer 1, having therefractive factor n₁ and the thickness d₁ and being not transparent atλ<λ₄ is applied on the disc.

The refractive indexes n₁, n₂, n₃, n₄, n₅ are selected so that to obtain

n₁≈n₂≈n₃≈n₄≈n₅

If necessary, layers 2 and 4 are not applied.

9. The thickness of layers d₁, d₂, d₃, d₄, d₅ are selected based on thefollowing:

d₂ is the minimum thickness under the condition of keeping thecontinuity, as a rule, d₂=0.05 μm.

d₄ is the minimum thickness under the condition of absorption of 99% ofthe incident radiation at λ<λ₄; as a rule, d₄=1 μm.

d₅ is the minimum thickness under the condition of F_(gl)>>F_(ad), whereF_(gl) is the force of gluing layer to layer, and F_(ad) is the force ofadhesion between the layer (1) and the photomask; as a rule, d₅=20-30μm.

d₁>d₄

d₃ is calculated based on the required number of dye molecules in orderto provide the positive registration of the minimal information pit.

The multi-layered optical fluorescent discs with photo-affined markersare produced similarly. The main difference lies in the fact that it isnecessary to perform an additional procedure of the wet treatment of theexposed photo-sensitive layer in order to remove non-fastened markers.The main advantage of this material is the possibility of using a widerange of water-soluble dyes with a photo-affined group introduced inthese dyes.

The production of multi-layered fluorescent discs, by using amaster-disc for the purpose of increasing productivity, can be based onthe application of the replicas. These replicas are made by stampingcopies from an ordinary master-disc on such material which is notsusceptible to the action of the polymer solvents used in theabove-mentioned method. In production of replicas is fully coincidentwith the production technology of the base for the DVD-type ordinarysingle-layer discs.

What is claimed is:
 1. The method of producing the multi-layered opticalfluorescent discs comprising the steps: producing a large number ofrelief-forming master-discs with the information recorded on the saiddiscs, said information being coded in the form of spirally-arrangedspatially-modulated micro-cavities (pits or grooves) for each of thelayers formed in the multi-layered optical disc; preparing liquidsolutions for an information luminescent medium (ILM); applying saidsolutions to the said master-disc; hardening the ILM solutions on thesaid master-disc; forming, in the said hardened layer, theinformation-modulated pattern in the form of the nodules capable ofemitting luminescence and filling the space of micro-cavities (pits orgrooves) in the said master-disc on the non-luminescent flat surface ofthe hardened layer of the ILM; removing said hardened layer of the ILMwith the luminescent micro-nodules on a thin polymer film; forming themulti-layered disc by means of sequential transfer of various hardenedILM from various master-discs on a stack of layers of a fluorescentoptical disc; and applying a protective coating on the first layer andapplying the multi-layered structure with the ILM on a substrate fromthe side of the last information layer.
 2. The method as in claim 1wherein the said master-disc is produced as a metallic-coated photomaskon a quartz or glass substrate with pits and grooves.
 3. The method asin claim 1 further including the following steps: cleaning the substratewhich is transparent in UV and visible spectrum segment; applying a thinmetal layer on the said substrate; applying a photo-resist to saidsubstrate; drying the photo-resist; exposing said photo-resist byfocused and modulated laser or electronic beam when scanning spirally;developing said photo-resist; etching the thin metal layer; removingsaid photo-resist; and depositing a metal layer by electroplating. 4.The method as in claim 3 wherein the thickness of the deposited metallayer is equal to the depth of the formed pits.
 5. The method as inclaim 3 wherein the thickness of the deposited metal layer is about0.35-0.5 μm.
 6. The method as in claim 3 wherein the surface of thelayer accumulated by electroplating exhibits non-wetting properties withrespect to the liquid solution of the ILM.
 7. The method as in claim 1wherein the liquid solution of ILM is photo-sensitive.
 8. The method asin claim 1 wherein the hardened ILM layers provide transmission on thewavelength of the activating radiation not exceeding 1% at the layerdepth equal to the depth of the said master-disc pit.
 9. The method asin claim 1 wherein, in order to provide a strong absorption on theactivation wavelength, additives with a high extinction factor areintroduced into the composition of the ILM to render it light sensitive.10. The method as in claim 1 wherein the composition of the ILMcomprises photo-chemically stable molecules of luminophor.
 11. Themethod as in claim 1 wherein the negative photoresists are included inthe ILM solutions to render them light sensitive.
 12. The method as inclaim 11 wherein the negative photoresists comprise thepolyvinyl-cyanamate derivatives.
 13. The method as in claim 1 whereinphotohardenable monomers or oligomers are used in the composition of theILM.
 14. The method as in claim 1 wherein photo-chemically stablepolymers are present in the ILM composition.
 15. The method as in claim1 wherein light sensitive non-luminescent substances capable of formingluminescent photo-products are introduced into the composition of theILM.
 16. The method as in claim 15 wherein lactam rhodamines or otherorganic dyes are used as the light-sensitive substance.
 17. The methodas in claim 1 wherein photo-active dyes are present in the ILM.
 18. Themethod as in claim 17 wherein lactone rhodamines or other organic dyesare the photo-active dyes.
 19. The method as in claim 1 whereinfluorescent markers fastened to polymer molecules sensitive to UVradiation are present in the ILM.
 20. The method as in claim 1 whereinazides are added as fluorescent markers to the ILM.
 21. The method as inclaim 1 wherein the formation of the information-modulated pattern codedin the form of spatially-modulated luminescent micro-cavities isperformed by illuminating the ILM layer by the actinic radiation fromthe side of the said master-disc.
 22. The method as in claim 1 whereinthe non-radiated portion of the ILM located on the said master-discsurface is removed by a solvent after being exposed to the actinicradiation.
 23. The method as in claim 1 wherein the substrate isnon-transparent.
 24. The method as in claim 23 wherein the surface ofthe substrate exhibits wetting properties with respect to the liquidsolution of the ILM.
 25. The method as in claim 1, wherein alight-insensitive composition is used as the liquid solution of the ILM.